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2011-10-11 13:32:09
RFC3339 instead of ISO8601, perhaps?

Of course, abusers will only pay attention to this if it benefits them and it's 
cheap to do so.  But they won't be distinguishable from legitimate clients that 
just don't know about this extension and retry by their own schedules, so one 
can't penalize such clients for not respecting the request.  On the other hand, 
you might be able to identify "good" clients (for some value thereof) by 
observing which ones do respect the request.

From: owner-ietf-smtp(_at_)mail(_dot_)imc(_dot_)org 
[mailto:owner-ietf-smtp(_at_)mail(_dot_)imc(_dot_)org] On Behalf Of Keith Moore
Sent: Tuesday, October 11, 2011 11:06 AM
To: Steve Atkins
Cc: SMTP Interest Group
Subject: Re: We need an IETF BCP for GREY LISTING

On Oct 11, 2011, at 1:45 PM, Steve Atkins wrote:

In that case, some minor extension to allow the SMTP server to communicate 
something a bit more nuanced than "Go away, come back later." might have some 

I could see value in that.  I could imagine an SMTP extension which, if 
included, indicated that the server might send a response of the form

4xx please retry between <date-time>-<date-time> response to say the MAIL (or maybe DATA) command, where <date-time> could 
be an ISO8601 date-time (the horror!) based on GMT (Z) and with no punctuation. 
 If the client included a SIZE the server could even do bandwidth reservation.  
Of course there would be no guarantee that the second attempt wouldn't result 
in some sort of 4xx response for other reasons.

Greylisting servers could certainly make use of it, though I don't know if it 
would be a good idea to recommend that greylisting servers use it.